1. Technical Field
The present invention relates generally to a pneumatic vehicle tire, and to a method and an apparatus for correcting at least one uniformity characteristic in the tire. In particular, the present invention relates to correcting the uniformity characteristic in the tire, such as radial force variation and/or conicity, without grinding any part of the tire.
2. Description of the Prior Art
It is known in the tire industry that it is difficult to manufacture a toroidal shaped pneumatic radial tire consistently the same every time from sheet and/or strip material. A typical pneumatic radial tire includes a pair of axially spaced apart and circumferentially inextensible beads. A carcass ply extends between the beads and is attached to a respective bead at axially opposite end portions of the carcass ply. The carcass ply includes a plurality of parallel extending reinforcing members. The carcass ply is formed into a toroidal shape and has a belt package located radially outward of the carcass ply in a crown portion of the tire. Tread rubber and sidewall rubber are applied over the belt package and carcass ply, respectively.
After the tire is assembled and cured, the tire is typically tested for a uniformity characteristic. "Uniformity" is defined herein as what a "perfect" or "ideal" tire would yield for certain measured characteristics when tested during rotation. "Uniformity characteristic" is defined herein as a deviation in those certain characteristics from what the perfect tire would yield during the testing.
Testing a tire for a uniformity characteristic typically begins with mounting the tire in a inflated condition on a test spindle of a uniformity tester. A test wheel is moved into engagement with the tire to radially deflect a portion of the tire a predetermined amount. The position of the axis of rotation of the test wheel relative to the axis of rotation of the tire is then fixed by a locking mechanism. The test wheel is rotated to cause rotation of the tire. Sensors associated with the test wheel sense radial and lateral loads transmitted by the tire to the test wheel during rotation of the tire.
One uniformity characteristic test which is generally performed on the tire is a test for radial force variation. Radial force variation is typically expressed as a variation in the force against the test wheel which is sensed during rotation of the tire. Radial force variation can be represented by a combination of first harmonic radial force variation through an Nth harmonic radial force variation or a composite radial force variation. The Nth harmonic is the last harmonic in a Fourier Series analysis of the composite radial force variation which is deemed acceptable to accurately define the radial force variation. It is known in the tire and automobile industries that vehicle ride is generally most affected by the first harmonic radial force variation of the tire. The first harmonic radial force variation is often associated with "radial runout" of the tire. Radial runout is defined as a difference in the radius from the axis of rotation to the outer periphery of the tire tread around the tire.
Another uniformity characteristic test which may be performed on the tire is a test for conicity. Conicity is defined as the tendency of a rotating tire to generate a lateral force regardless of the direction of rotation of the tire. Conicity is expressed in terms of average lateral force generated during rotation in both directions of the tire against the load.
Such uniformity characteristics may be attributed to the manufacture of a tire from the sheet and/or strip material. The uniformity characteristics can simplistically be viewed as a deviation from perfect roundness of the outer circumference of the tire, as deviation from spindle load transmitted by a perfect tire during rotation (radial force variation) or as deviation from straight tracking during rotation (conicity). For example, the tread rubber of the tire may be thicker or thinner in one location around the outer circumference of the tire. There may also be areas of the tire having increased strength because of a doubling of a tire reinforcement, such as at the splice from sheet carcass ply material. Lack of bead concentricity of the tire may also be a problem. The beads of the tire may be not exactly concentric relative to the axis of rotation of the tire or the tread may not be concentric with the beads (radial runout). The carcass ply of the tire may be subjected to more or less localized stretch of the carcass reinforcing members during assembly of the tire. The molding and curing processes of the tire assembly could also create localized stretching of the carcass reinforcing members. The belt package of the tire may be axially displaced or conically shaped.
If the uniformity characteristic of the tire has a magnitude which is less than a predetermined relatively low minimum magnitude, which is deemed not to be detrimental to a vehicle ride or produce undesirable vibrations in the vehicle, the tire may be shipped to a customer. If the uniformity characteristic magnitude is greater than a predetermined maximum threshold magnitude, the tire is scrapped. If the uniformity characteristic magnitude is between the relatively low minimum magnitude and the maximum threshold magnitude, the tire may be suitable for correction.
Typically, prior art correction of a uniformity characteristic of a tire, such as radial force variation, included grinding of tread rubber about the outer circumference of the tire at a selected location and up to 180 degrees about the outer circumference of the tire. However, grinding of the tire has certain disadvantages. For example, grinding can contaminate a tire plant environment, reduce the useful tread life of the tire or may render the tire visually unappealing. Prior attempts at correcting a pneumatic tire uniformity characteristic without grinding are disclosed in U.S. Pat. Nos. 3,529,048; 3,632,701; 3,838,142; 3,872,208; 3,880,556; 3,945,277 and 5,060,510.
U.S. Pat. No. 3,529,048 discloses placing a tire on a fixture immediately after the tire is removed from a mold and before it is cooled. The tire is inflated to its recommended operating pressure. A radial load is applied to the tire and the tire is rotated for a time at least equal to the tire cure time. The flexing of portions of the tire allow components or portions of the components of the tire to "relatively move" before the tire is completely cured to yield uniform stresses in the components.
U.S. Pat. No. 3,632,701 discloses heating a tire after curing to a temperature elevated above an ambient temperature. The elevated temperature is maintained for about sixty minutes while the tire is inflated to a pressure of up to 50 psi. This obviously has drawbacks in a modern tire production plant because of the relatively long time required to correct the uniformity characteristic of the tire compared to a cure cycle time of less than thirty minutes for a passenger car radial tire.
U.S. Pat. No. 3,838,142 discloses subjecting selected sections of the tire to radiation to increase the modules of elasticity of those sections. U.S. Pat. Nos. 3,872,208 and 3,880,556 disclose applying heat to a portion of the inner surface of the tire. U.S. Pat. No. 3,945,277 discloses applying heat to the tire sidewalls during rotation of the tire in contact with rollers in order to "condition" the tire.
U.S. Pat. No. 5,060,510 discloses correcting radial force variation of a tire and rim assembly without grinding the tire tread. A pair of circumferential shims are placed between respective tire bead areas and mounting areas of the rim as a function of the measured radial force variation. Each shim has a varying thickness over its circumference. For a flat seat rim, the largest thickness portion of the shims are placed at the location of the largest amplitude of the radial force variation.